Conventionally, various printing devices have been proposed in which an image is displayed by a liquid crystal display element (hereinafter, referred to as LCD (Liquid Crystal Display) and light from a light source is directed to a photosensitive material through this LCD so as to print the image. In the printing device of this type, for example, a film image is acquired as image data consisting of a plurality of pixels, and pixels on the LCD are modulated in accordance with the image data thus acquired so that the photosensitive material is exposed.
In the above-mentioned conventional printing device, in the case when a color image is printed on a photosensitive material, color exposure is carried out based upon the so-called additive color process. In other words, the system which represents a color image by superposing light rays having blue (B), green (G) and red (R) components is adopted.
The following description will discuss a structural example of the conventional printing device. FIG. 7 is a perspective view that schematically shows the construction of the printing device which is provided with an LCD and carries out face exposure. Here, the printing device having such a construction is referred to as a printing device of one-head face exposure system.
This printing device of one-head face exposure system is provided with a light source 51 for emitting white light, a BGR filter 52 having filters corresponding to the respective colors of blue, green and red, an LCD 53 and a printing lens 54. Light, released from the light source 51, is made incident on the LCD 53 after passing through any of the filters of the respective colors in the BGR filter 52. The light, made incident on the LCD 53, is modulated by the LCD 53 that displays image information corresponding to a color at this time, and then directed onto printing paper 55 as a photosensitive material through the printing lens 54. An image as a whole to be printed is displayed on the LCD 53, and the image is printed on the printing paper 55 that is placed at rest.
As described above, in the printing device of the one-head face exposure system, color exposure is carried out by using the BGR filter 52. In other words, for example, when the filter corresponding to blue is being used, image information corresponding blue is displayed on the LCD 53 so that an exposing process of the blue component of the image is carried out on the printing paper 55, and the same exposing processes of the green and red components are then carried out in this order so as to print the color image.
FIG. 8 is a perspective view that schematically shows the construction of a printing device which is provided with light sources and LCDs that correspond to the respective colors of blue, green and red, and carries out face exposure. Here, the printing device having such a construction is referred to as a printing device of three-head face exposure system.
This printing device of three-head face exposure system is provided with three light sources 51B, 51G and 51R for respectively emitting blue light, green light and red light, three LCDs 53B, 53G and 53R, a composition prism 56 and a printing lens 54. Light rays, released from the light sources 51B, 53G and 53R, are made incident on the corresponding LCDs 53B, 53G and 53R, and modulated in accordance with image information for the respective colors. The resulting light rays, released from the LCDs 53B, 53G and 53R are composed by the composition prism 56, and projected onto printing paper 55 through the printing lens 54. An image as a whole to be printed is displayed on each of the LCDs 53B, 53G and 53R for each of the colors, and the image is printed on the printing paper 55 that is placed at rest.
As described above, in the printing device of the three-head face exposure system, the light sources 51B, 51G and 51R and the LCDs 53B, 53G and 53R are used for directing image light rays of the blue, green and red components to the composition prism 56 simultaneously so that an exposing process is carried out on the printing paper 55 by using color image light rays from the composition prism 56.
FIG. 9 is a perspective view that schematically shows the construction of a printing device which is provided with light sources and LCS's (Liquid Crystal Shutters) that correspond to the respective colors of blue, green and red, and carries out scanning exposure by directing parallel light rays from the respective LCS's onto printing paper. In this case, LCS refers to a liquid crystal element in which respective pixels are aligned in one row or in several rows. Here, the printing device having such a construction is referred to as a printing device of three-head parallel scanning exposure system.
This printing device of three-head parallel scanning exposure system is provided with three light sources 51B, 51G and 51R for respectively emitting blue light, green light and red light, three LCS's 57B, 57G and 57R, and three rod lens arrays 58B, 58G and 58R. Light rays, released from the light sources 51B, 51G and 51R, are made incident on the corresponding LCS's 57B, 57G and 57R, and modulated in accordance with image information for the respective colors. The resulting light rays, released from the LCS's 57B, 57G and 57R are projected onto printing paper 55 through the respective rod lens arrays 58B, 58G and 58R. Image data corresponding to one row or several rows of an image to be printed is displayed on the LCS's 57B, 57G and 57R for the respective colors, and scanning exposure is carried out by transporting the printing paper 55 in a direction orthogonal to the length direction of the respective LCS's.
As described above, in the printing device of the three-head parallel scanning exposure system, the light sources 51B, 51G and 51R and the LCS's 57B, 57G and 57R are used for directing image light rays of the blue, green and red components onto the printing paper 55 in parallel with each other. Then, the display timing of the LCS's 57B, 57G and 57R is controlled based upon the relationship between the distance between irradiation positions of the respective colors and the transporting speed of the printing paper 55 so that a color exposing process is carried out on each of the respective pixels of the image to be printed.
FIG. 10 is a perspective view that schematically shows the construction of a printing device which is provided with light sources and LCS's (Liquid Crystal Shutters) that correspond to the respective colors of blue, green and red, and carries out scanning exposure by directing light composed by light rays from the respective LCS's onto printing paper. Here, the printing device having such a construction is referred to as a printing device of three-head composition scanning exposure system.
This printing device of three-head composition scanning exposure system is provided with three light sources 51B, 51G and 51R for respectively emitting blue light, green light and red light, three LCS's 57B, 57G and 57R, a composition prism 56 and a rod lens arrays 58. Light rays, released from the light sources 51B, 51G and 51R, are made incident on the corresponding LCS's 57B, 57G and 57R, and modulated in accordance with image information for the respective colors. The resulting light rays, released from the LCS's 57B, 57G and 57R are composed by the composition prism 56, and projected onto printing paper 55 through the respective the rod lens array 58. Image data corresponding to one row or several rows of an image to be printed is displayed on the LCS's 57B, 57G and 57R for the respective colors, and scanning exposure is carried out by transporting the printing paper 55 in a direction orthogonal to the length direction of the respective LCS's.
As described above, in the printing device of the three-head composition scanning exposure system, the light sources 51B, 51G and 51R and the LCS's 57B, 57G and 57R are used for directing image light rays of the blue, green and red components onto the composition prism 56 simultaneously, and the resulting color image light rays from the composition prism 56 are used for exposing the printing paper.
In the case of the printing device of one-head face exposure system as shown in FIG. 7, a color image is printed by carrying out serial exposing processes in the order of blue, green and red colors as described above. Therefore, as compared with the arrangement for carrying out exposing processes with the respective colors simultaneously, this system has a problem of low processability.
In the case of the printing device of three-head face exposure system as shown in FIG. 8, as described above, image light rays of the blue, green and red components are composed by using the composition prism 56, and the light rays corresponding to the respective colors are simultaneously directed onto the printing paper 55. In this manner, when the composition prism 56 is used, the aligning positions and aligning directions of the composition prism 56 and the LCDs 53B, 53G and 53R need to be set very accurately so as to keep an accurate positional relationship between the respective pixels representative of the respective color components of the image light rays; this causes an increase in production costs as well as manufacturing time. Moreover, the composition prism 56, which is a comparatively expensive member, also causes an increase in material costs.
In the printing device of three-head parallel scanning exposure system as shown in FIG. 9, as described above, a scanning exposure process is carried out while the image light rays of the blue, green and red components are directed to respectively different positions on the printing paper 55. In this arrangement, the image display timing of the LCS's 57B, 57G and 57R has to be controlled precisely so that the respective color components of the pixels in an image to be printed on the printing paper 55 are accurately superposed on each other. In other words, data corresponding to the respective pixels need to be displayed in different timing for the respective color components; this makes the image controlling process more complex.
Moreover, the image display timing is set based upon the relationship between the gap of the light rays of the respective color components and the transporting speed of the printing paper 55; however, in the event of an error in these values, pixel offsets, which are caused by offset exposing light rays of the respective color components, tend to occur in the respective pixels. Moreover, in the case when the aligned position of the optical system is offset with respect to the image light rays of the respective color components, the above-mentioned pixel offsets tend to occur. In other words, the system of this type is highly susceptible to the occurrence of pixel offsets.
In the printing device of three-head composition scanning exposure system as shown in FIG. 10, as described above, the image light rays of the blue, green and red components are composed by the composition prism 56, and while simultaneously projecting the light rays of the three colors onto printing paper 55, scanning exposure is carried out. As described above, when the composition prism 56 of this type is used, the aligning positions and aligning directions of the composition prism 56 and the LCS's 57B, 57G and 57R need to be set very accurately so as to keep an accurate positional relationship between the respective pixels representative of the respective color components of the image light rays; this causes an increase in production costs as well as manufacturing time. Moreover, the composition prism 56, which is a comparatively expensive member, also causes an increase in material costs.